This invention relates to modular connector structures that are shaped to receive the edges of printed circuit boards and that have contacts spaced a unit distance apart so that they can engage, separately, each of the contact pads, or fingers, spaced apart by the same unit distance along the surfaces of the boards. In particular, it relates to connector structures that have conversely shaped interlocking parts at their ends to interlock end-to-end with similar structures to form a substantially self-supporting connector that can have any desired number of contacts, each spaced an integral multiple of the same unit distance from all of the contacts on all of the modules.
U.S. Pat. No. 3,340,440 of Jerry B. Minter describes a connector, or clip, made of round, resilient wire bent in M-shaped configuration with a U-shaped central part into which the edge of a printed circuit board can be smoothly inserted. The sides of the U-shaped central portion are formed so that they are closest together at a location that is a little distance away from the bight that defines the central bottom of the U. The distance between the side members at the region of closest approach is a little less than the thickness of the printed circuit board so that a small part, called a contact area, of each side member will press firmly against the printed circuit board to maintain good mechanical engagement with the board and good electrical connection with a finger printed at that location along the edge of the board. Because the wire is round and is curved at the contact areas, the surface configuration of each contact area is essentially spheroidal, although of very small lateral extent. The smoothness of the curvature in all directions allows the board to be slid into position without scratching off the metal that forms the contact finger at that location, yet the small lateral extent of the contact area concentrates the resilient force upon the finger to a large number of newtons per square centimeter, which is sufficient to wipe away contamination that may have accumulated on the contact area of the wire or on that part of the finger engaged by the wire.
The most popular configuration of the clips in U.S. Pat. No. 3,340,440 is M-shaped, with the U-shaped portion at the center, and the ends of the sides curved back in the opposite direction and extending parallel to each other far enough beyond the bight of the U-shaped portion to be able to fit into correspondingly spaced holes in a support member. The support member is normally another printed circuit board and is frequently called a mother board. The printed circuit board that fits into the U-shaped portion and is thus supported by the mother board is frequently referred to as a daughter board. One of the advantages of the M-shaped clip is that any desired number of them can be supported in a pair of parallel rows of holes in a mother board to engage the same number of contact pads, or fingers, along the edge of a daughter board. The clips are held in place by being soldered to pads surrounding each of the holes on the mother board. Unlike other printed circuit board receptacle connectors that are made in standard sizes having only certain numbers of contacts supported in a molded plastic trough, a connector formed only by the wire clips would not have the extra weight nor occupy the extra volume of the plastic trough nor would it have more contacts than necessary.
The individual M-shaped clips do require somewhat delicate and careful handling when their two parallel ends are being inserted into the proper holes in the mother board. To overcome that problem, another Minter U.S. Pat. No. 3,940,849, provides a method of handling the clips by means of a carrier in which a number of clips can more easily be placed in alignment so that they can all be inserted simultaneously in the mother board. The carriers may be placed end to end to allow any number of clips to be mounted on the mother board at one time, but the carriers must be removed after the clips have been soldered in place. The reason is that each carrier has a central wall that occupies the space that will later be occupied by the daughter board.
The fact that both sides of the U-shaped central portion of the M-shaped clip firmly engage the daughter board means that such clips cannot be used with daughter boards that have fingers on both surfaces adjaent the same edge if, as is frequently the case, the fingers on one surface are supposed to be electrically isolated from those on the opposite surface. To overcome that problem, Minter has described and claimed in his copending U.S. patent application Ser. No. 596,096 clips that are not M-shaped but are more or less J-shaped.
A problem common to all multi-contact printed circuit board edge connectors is that, in order to establish satisfactory electrical connection, each contact must exert sufficient pressure on the respective finger of a printed circuit daughter board, but that pressure causes frictional drag during insertion and removal. As a result, a very high force may be required to insert into the connector a daughter board that has a large number of contact fingers.
To reduce the insertion force, Minter U.S. Pat. No. 4,327,955 describes a structure in which each M-shaped clip, or each pair of clips referred to in that patent as U-shaped is placed in a depression in one surface of a wafer made of insulating material that has a certain overall front-to-back thickness. A multi-contact connector to receive the edge of a daughter board can then be made by assembling a stack of such wafers coresponding in number to the desired number of contacts and then clamping the stack together Each wafer has two channels through it to permit two rods to be pushed therethrough to engage all of the enclosed clips. These rods reduce insertion force by either forcing the clips away from the path of an incoming daughter board until it is in place or by pressing the clips against the daughter board after it is in place in the connector.
A problem that is not immediately obvious with a stack of wafers, each holding only a single contact layer consisting of a single M-shaped clip or a pair of the U-shaped clips arranged in mirror image fashion, is that the thickness of the wafers is not precisely the same from wafer to wafer, and since the thickness determines the spacing between successive contact layers, it is possible for a batch of wafers that are very slightly too thick to create a cumulative error that will cause one or more contacts at the far end of the stack not to engage the proper finger. The possibility of such an error places too low a limit on the maximum number of wafers that can be stacked together. In addition, the only thing holding the stack together is a clamp; there are no conversely shaped interlocking parts on opposite ends of each wafer to allow them to be held together as a self-supporting multi-contact connector.